1. Field of the Invention
This invention relates to nuclear fuel storage racks and more particularly to a lateral support and preload device for absorbing relative motion and maintaining seismic support between the racks and their containing enclosure.
2. Description of the Prior Art
Nuclear power generating plants are typically fueled by elongated fuel assemblies, such as those including a bundle of nuclear fuel rods. Subsequent to utilization of the fuel assemblies in a nuclear core they are stored within fuel racks positioned within an enclosure such as a spent fuel pit.
A typical spent fuel pit includes leak-tight walls and a floor made of concrete and other support materials lined with stainless steel. The fuel racks include a rectangular closely spaced array of cells, each cell sized to receive a fuel assembly. Because the assemblies have been irradiated they must be shielded, and water is typically utilized for this purpose. The assemblies are therefore maintained submerged within water within the pit.
In the prior art the fuel racks have been supported in several manners. Typically, adjacent racks are rigidly affixed to one another. In many installations, the racks are further bolted to the pit floor, without lateral support to the fuel pit walls. In other installations, the affixed racks sit on adaptor plates on the pit floor, and are laterally supported through rigid supports between the peripheral racks and the pit walls. Such support arrangements, however, do not provide an ideal response under assumed large seismic accident conditions.
The former, or purely cantilever type support, could result in excessive rack deflection under seismic loading, damaging the contained assemblies. Depending upon the spacing of the rack with respect to the fuel pit walls, excessive deflections could also result in undesirable rack and wall impact. The latter arrangement can also result in excessive deflections, unless very large lateral supports or a large number of lateral supports are utilized along the rack height. Such supports are undesirable where lack of space within the pit is a significant factor, particularly in those installations where back-fit of additional storage capacity or support is contemplated.
Most importantly, however, the seismic response of these lateral supports can be detrimentally lessened over a period of time as a result of relative motion of the fuel pit walls and the racks. This motion can occur, for example, as a result of thermal expansions and contractions of the structures. Such motion can relax the lateral support and can cause spatial gaps to form between the supports and the racks or walls. These gaps could cause excessive impacts under accident conditions. If the supports are rigidly affixed to both the peripheral racks and the walls, the relative motion can produce undesirable stresses on the structures.
Some additional lateral supports have been proposed which overcome these deficiencies through use of preload devices which, however, are to be positioned through an arclike motion between the racks and the walls. Such motion, however, makes the preload difficult to adjust and the devices difficult to be properly and accurately positioned. They also require a significant amount of space between the racks and the walls in order to accommodate the arc motion. And, in addition to their relatively high cost, the arc motion devices are difficult to properly seat because of the high frictional forces and slipping among contact surfaces that installation of such devices provides.
It therefore will be recognized that a lateral support system which provides proper lateral support in response to a seismic occurrence which also provides a sufficient mechanism, such as a preload to accommodate relative motion between the racks and the pit walls over a period of time, is desirable. Further, because the racks remain under water, it is desirable to provide such a system that can be remotely positioned and maintained. It is also desirable that such systems be compact, easily installed, relatively inexpensive, and independent of slippage among contact surfaces.